The present invention relates to amplifying circuits and more particularly to an amplifying circuit in which the gain in the predetermined frequency band is variable.
FIG. 1 shows a circuit diagram of a conventional amplifying circuit. An amplifying circuit 20 shown in FIG. 1 is configured such that the gain in the low frequency band is variable, and is used, for example, as an amplifying circuit for amplifying reproduced audio signals in a stereo headphone.
An amplifier 21 of FIG. 1 has a frequency characteristic as shown in FIG. 2A where the voltage gain A.sub.V1 drops beyond a low cut-off frequency f.sub.L determined by a resistance R.sub.4 and a capacitor C.sub.2. The output voltage of the amplifier 21 is fed to the input of a summing amplifier 22 via a variable resistor R.sub.5.
The voltage gain A.sub.V2 of the amplifier 23 has a flat frequency characteristic extending into the high-frequency range, as shown in FIG. 2B. The output voltage of the amplifier 23 is fed to the input of the summing amplifier 22 and added to the output voltage of the amplifier 21.
Thus, the frequency characteristic of an output voltage V.sub.OUT from the amplifier 22 obtained at an output terminal 24 is as shown in FIG. 2C. That is, the amount of boosting of the output voltage gain in the low-frequency band can be adjusted as indicated by an arrow A in FIG. 2C by adjusting the variable resistor R.sub.5. In this way, the circuit of FIG. 1 enables an audio effect to be adjusted so as to be suitable for the user's taste and the type of music source.
However, the above-mentioned amplifying circuit 20 has a problem in that, when the amount of boosting of the output voltage gain in the low-frequency band is altered, a medium cut-off frequency (a cut-off frequency in the intermediate frequency band) F.sub.M is also altered as indicated by an arrow F in FIG. 2C, thereby causing an unfavorable audio effect. Another problem with the conventional circuit of FIG. 1 is that there are a comparatively large number of amplifiers required, thus adding a substantial expense to the circuit.